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Why is powder caking an important phenomenon for industrial applications?

Learn more about powder caking with our new blog article.

What is powder caking ?

The macroscopic behaviour of powders is affected by many factors: particle properties (shape, size distribution, chemical composition), interparticle cohesive forces, friction between the particles, state of stress of the powder, environmental conditions (temperature, relative humidity). However, powder behaviour can change with time. Indeed, strength of granular materials can increase if stored under a compressive stress for a long-time interval. This is the case, for example, of powder stored within a silo. Such a phenomenon is called Caking or time consolidation. Caking can be affected by adhesive forces. For example: 1) solid bridges due to crystallization induced by drying of moist bulk solids; 2) bridges due to the sintering of powders, e.g. polymeric powders stored at temperatures not not much lower than the melting temperature (Lumay et al., 2020); 3) plastic deformation at the contact points between the particles, which causes the increase of adhesive forces through the increase of the contact areas between the particles; 4) chemical reactions between the particles; 5) biological processes, such as the fungal growth on biologically active materials (Schulze, 2021).

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Caking of powders is a serious problem in various industries, such as food, chemical and pharmaceutical industries. For example, caking of food powders occurs during processing, handling, and storage and can cause agglomeration, deterioration of organoleptic quality, shortened shelf-life and reduced product quality. Caking could determine blockages in pharmaceutical tableting machines (Listiohadi et al., 2008) or can cause bad content uniformity in a pharmaceutical formulation (Neveu et al., 2020). In case of a powder stored within a silo, particles are compressed due to the gravity force acting on the powder column itself or by external forces. The larger the compressive force on the powder, the larger are expected to be the cohesive forces between the particles because of increase of contact surface. In this situation, cohesive forces due to caking are detrimental because can prevent powders from flow out from storage container. In all these cases, powder caking impacts negatively the product manufacturability and quality. Therefore, it is associated with additional costs due to the stop of the production process or due to further processing to regain a free-flowing powder (Carpin et al., 2016).

However, there are some processes in which caking can be beneficial. For example, in some dynamic processes like in the mixing systems, granulation or clumping of particles could be an advantage because the flow properties of the powder could improve due to bigger granule sizes. Caking is also useful when it is necessary to press powders to create pills or briquettes.

Consequently, case by case, it is necessary to investigate about the effect of caking on macroscopic powder behaviour. To this aim it is necessary to predict the powder flowability in conditions as close as possible to the process in which the powder is used. Therefore, proper characterisation instruments need to be adopted, which are able to accurately investigate powder caking at different conditions, both static and dynamic.

Evaluation of powder caking by using Granutools' instruments

Granutools provides instruments that can help in doing these evaluations by analysing powder flowability in different conditions: from very low to very high shear rates. For example, the GranuPack is able to quantitatively characterise powder flowability (Hausner ratio, packing dynamic, among others) in quasi-static conditions, in simple, fast and intuitive ways, with repeatable measurements, with easy cleaning, within a closed system for safety requirements, and with a specific initialization protocol to remove user independency in order not to affect the test results. The GranuPack can be used to investigate powder caking by looking, for example, at the effect of caking on changes of packing kinetic (Lumay et al., 2020) as well as on variations in initial and final bulk density (after a fixed number of taps). The GranuPack High-Temperature instrument allows to investigate the effect of storage temperature (up to 200°C) on the packing dynamics, e.g. the effect of onset of solid bridges due to pre-sintering or caking on the packing behavior of the powder. The GranuDrum is a very versatile instrument, by means of which it is possible to characterise powder flowabality (Dynamic Angle of Repose, Cohesive Index, among others) in low compressive state and at shear rates from low to high, thus providing useful information on the flowability of powders in very different processes, such as the the recoater process in additive manufacturing, but also the rotating mixer in a pharmaceutical process. The GranuDrum can be used to study the effect of caking on thixotropic powder behaviour, for example, in terms of change of powder cohesiveness (Cohesive index) at different shear stresses. The GranuCharge instrument allows the determination of powder initial charge and after a flow in contact with different materials (e.g. PVC, Teflon, Stainless steel) to investigate the tribocharging phenomenon, which could drastically influence the material flow properties. Caking can change the charge build-up of powders, due to reduction of exposed powder surface area for tribocharging.

By using these characterisation instruments, the user should be able to predict whether a powder can flow from a storage container, how its flowability changes due to caking and how it could behave during its transportation.

How to evaluate powder caking with GranuPack

References

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